Holy cow, I wrote a book!
Why does the Welcome screen beep if you hit the space bar
after the computer has finished starting up?
For compatibility with a side effect of the Windows XP Welcome screen.
A beta tester asked us why we removed the beeps that were generated
if you started typing at the Welcome screen after the computer finished
Well, we didn't remove the beeps consciously.
That the old Welcome screen beeped was just a side effect of
the way the old Welcome screen positioned focus at startup and
how it handled unexpected input.
When the Welcome screen was rewritten for Windows Vista,
focus and input were handled differently,
and it so happened that the new way
didn't beep under the initial conditions.
The tester in this case was blind and relied upon the beeping to
know when the system was ready.
The Welcome screen folks had to go back and take extra care to
ensure that if you start the computer and wait for the Welcome
screen, then hit the space bar, you get a beep.